(1) Field of the Invention
The present invention relates to a process for production of propylene glycol in high yield and selectivity in an aqueous reaction mixture of lactic acid and hydrogen with an essentially pure elemental ruthenium catalyst on an inert support at elevated pressure and temperature. In particular, the present invention relates to the process wherein the catalyst is a ruthenium salt deposited on a microporous support, reduced to ruthenium metal on the support with hydrogen, and oxidized with oxygen to provide a ruthenium oxide surface on the ruthenium metal and wherein the catalyst is maintained in the surface oxidized state until it is reduced with hydrogen prior to the reaction process.
(2) Description of Related Art
Propylene glycol is a nontoxic chemical used in polymer production; as a nontoxic antifreeze; in food, drinking, cosmetics, and pharmaceutical applications. In the United States, the production rate of propylene glycol is approaching one billion pounds per year, with growth projected at 5 to 7% per year.
Commercial production of propylene glycol is currently petroleum-based and involves the high pressure and high temperature hydrolysis of propylene oxide. Propylene oxide is manufactured by either the chlorhydrin process or the per-oxidation process (Szmant, H. H., Organic Building Blocks of the Chemical Industry, Wiley, New York, N.Y. pp. 281-283 (1989)). Making either propylene glycol precursor is a multi-step process that involves a variety of organic solvents and results in the formation of multiple byproducts. The selling price of propylene glycol using these petroleum-based precursors is about $0.60 to $0.65 per pound.
Alternative processes for producing propylene glycol have been reported. For example, the hydrogenation of organic esters to alcohols and glycols was reported by Adkins and co-workers who were able to achieve 80% yields of propylene glycol from methyl lactate over copper/chromium oxide and Raney nickel catalysts at 150xc2x0 to 250xc2x0 C. and extremely high hydrogen pressures of about 20 to 30 MPa (2,900 to 4,350 PSIG) (Bowden and Adkins, J. Am. Chem. Soc. 56: 689 (1934); Adkins and Billica, J. Am. Chem. Soc. 70: 3118 (1948); Adkins and Billica, J. Am. Chem Soc. 70: 3121 (1948)). In addition to high pressures, high catalyst loadings were necessary as well to achieve these relatively high yields. Broadbent et al. (J. Org. Chem. 24: 1847 (1959)) was able to obtain propylene glycol yields as high as 80% from ethyl lactate at 150xc2x0 C. over rhenium black catalysts, but at very high hydrogen pressures of about 25 MPa (3,626 PSIG). More recently, U.S. Pat. No. 5,731,479 to Antons discloses a process for preparing optically active alcohols from optically active carboxylic acids with hydrogen in the presence of ruthenium catalysts. An example is provided for the conversion of optically active L-(+)-lactic acid to L-(+)-propane-1,2-diol. The yields obtained were about 88% over bulk ruthenium powder, and 74% over ruthenium supported on carbon. To achieve efficient conversion of an optically active carboxylic acid to its corresponding optically active alcohol, the process is performed at a relatively high pressure (10 to 20 MPa) and the reaction is allowed to proceed to completion.
While the aforementioned relate to the conversion of lactic acid and derivatives to propylene glycol, there are several U.S. Patents related to the aforementioned which disclose various mixed bed catalysts and processes for their use in hydrogenation reactions for converting organic acid esters to their alcohols.
U.S. Pat. Nos. 4,985,572 and 5,149,680 to Kitson et al discloses a process for catalyzed hydrogenation of carboxylic acids and their anhydrides to produce the corresponding alcohol and/or carboxylic acid ester and catalysts for achieving the conversion. The process is particularly applicable to the hydrogenation of dicarboxylic acids and their anhydrides using catalysts that comprise an alloy of a noble metal from Group VIII of the Periodic Table of Elements and at least one metal capable of alloying with the Group VIII noble metal.
U.S. Pat. No. 5,478,952 to Schwartz discloses improved hydrogenation catalysts consisting essentially of highly dispersed, reduced ruthenium and rhenium on carbon supports. Specifically, the catalyst relates to the production of tetrahydrofuran, gamma-butyrolactone, 1,4-butanediol and the like from a hydrogenatable precursor such as maleic acid, succinic acid in an aqueous solution in the presence of hydrogen and one of the above catalyst.
Finally, U.S. Pat. No. 5,536,879 to Antons et al. discloses a hydrogenation process which uses ruthenium catalysts to prepare optically active amino alcohols from optically active amino acids. The process is performed at relatively high pressures to achieve efficient conversion of the amino acid to its corresponding alcohol.
The catalysts used in the above related patents can be adapted to convert lactic acid to propylene glycol. However, the disclosed hydrogenation reactions generally have inadequate yields and selectivities to propylene glycol. Inadequate selectivity results in the production of unwanted alkanes, such as methane and ethane, by side reactions. Because these byproducts reduce overall yield and contaminate the final product, the byproducts increase the overall costs associated with producing propylene glycol from lactic acid. Therefore, commercial production of propylene glycol remains dependent on petroleum-based routes for its synthesis.
Thus, there remains a desire to provide a low-cost process for producing propylene glycol from renewable resources. In particular, it would be desirable that the process enable the bulk production of propylene glycol in a process that can be performed under relatively mild conditions which results in high yields and selectivity for propylene glycol.
The present invention provides a process for the low-cost, bulk production of propylene glycol from lactic acid. The process produces high yields of propylene glycol from xe2x80x9ccrude or unrefinedxe2x80x9d lactic acid under reaction conditions with high selectivity and, therefore, low byproduct formation. Thus, the present invention provides a process for the production of propylene glycol, which comprises: reacting a reaction mixture of lactic acid and hydrogen in water with a ruthenium catalyst on an inert support with a BET surface area between about 1 to 1,000 m2 per gram at a hydrogen pressure between about 3.4 to 16.5 MPa (500 to 2,400 PSIG) and a temperature between about 500 to 200xc2x0 C., more preferably between 1000 and 170xc2x0 C.; and removing the propylene glycol from the reaction mixture.
The present invention also provides a composition which comprises an essentially pure elemental ruthenium on an inert microporous support, wherein the ruthenium is deposited on the support by drying a water solution of the ruthenium salt on the support, reducing the salt to the elemental ruthenium on the support with hydrogen, and then reacting the ruthenium on the support with oxygen to provide oxide of the ruthenium on surfaces of the ruthenium. Preferably, the ruthenium is deposited on an inert microporous support selected from the group consisting of alumina, titania, silica, alumina silicate, and microporous carbon.
The process of the present invention is preferably conducted at hydrogen pressures of 3.4 to 8.3 MPa (500 to 1,200 PSIG) in a continuous or batch process. When the process is a batch process, the reaction mixture is in a closed vessel provided with a stirrer which agitates the reaction mixture. When the process is a continuous process, the continuous process is a preferably a fixed bed reactor and the reaction mixture is trickled through a bed of the catalyst. Further, in the continuous process the gas flow can either be up or down. In a preferred embodiment, the hydrogen gas mixture is passed upward through the bed and the aqueous reaction mixture flows downward. In the continuous process it is preferable that the reaction mixture has a space velocity of between about 0.5 and 5 grams of lactic acid per gram of the catalyst per hour. In either the batch or continuous process, the reactor temperatures are preferably between about 500 and 200xc2x0 C., and more preferably between 100xc2x0 and 170xc2x0 C., for maximum results. Finally, to reduce the formation of reaction byproducts, the reaction mixture can further comprise sulfur added either to the liquid or gas phase.
The present invention further provides that the lactic acid can be provided to the reaction as a fermentate produced by a microorganism or other crude or unrefined preparation. In general, when the lactic acid is produced be a microorganism, the lactic acid may be converted to its salt during processing of the fermentate which can then be acidulated. Therefore, when the lactic acid is provided in a solution as its salt or a partially acidulated salt, this xe2x80x9ccrude or unrefinedxe2x80x9d reaction mixture may further comprise H2SO4 or other acid. Preferably, the H2SO4 is added to the lactic salt solution to provide a 1 to 1 molar ratio of lactate salt to acid equivalent and the anion, other than lactate is substantially removed. In general, the lactic acid so produced is provided at a concentration between about 5% to 90% by weight of the reaction mixture.
Thus, in a preferred embodiment, the present invention provides a process for preparation of propylene glycol comprising: (a) providing a ruthenium metal catalyst prepared by reacting hydrogen gas with a ruthenium salt deposited and dried on an inert microporous support having a BET surface area between about 1 and 1,000 m2 per gram; (b) drying the catalyst in an oxygen containing atmosphere so as to provide oxide of the ruthenium on surfaces of the ruthenium; (c) introducing the dried catalyst into a reaction vessel; (d) reacting the catalyst with the oxide on the surface with hydrogen in the vessel; (e) reacting in the vessel at elevated pressures and at elevated temperatures, a reaction mixture of a lactic acid in water in the presence of the catalyst, wherein the temperature is between about 50xc2x0 to 200xc2x0 C., preferably between 100xc2x0 and 170xc2x0 C. and the hydrogen pressure is between about 3.4 to 16.5 MPa (500 to 2,400 PSIG) to produce the propylene glycol in the reaction mixture; and (f) recovering the propylene glycol from the reaction mixture. In a preferred embodiment, the ruthenium is deposited on a support selected from the group consisting of alumina, titania, silica, alumina silicate, and microporous carbon. To reduce the formation of reaction byproducts, the reaction mixture can further comprise sulfur.
In the foregoing processes of the present invention, the propylene glycol is recovered in a yield of at least 80%, preferably in excess of 90%, and a selectivity to propylene glycol of at least 80%, preferably in excess of 90%.
The present invention further provides a composition which comprises an essentially pure elemental ruthenium on an inert microporous support, wherein the ruthenium is deposited on the support by drying a water solution of ruthenium salt on the support, reducing the salt to elemental ruthenium on the support with hydrogen, and passivating the composition in an oxygen containing atmosphere so as to provide an oxide of the ruthenium on surfaces of the ruthenium. Preferably, the inert microporous support is selected from the group consisting of alumina, titania, silica, alumina silicate, and microporous carbon. In a preferred embodiment, the inert microporous support has a BET surface area between about 1 to 1,000 m2 per gram, more preferably between about 100 to 1,000 m2 per gram. Preferably, drying the ruthenium salt on the catalyst is at about 25xc2x0 C. for about 5 hours and then under a vacuum of about 30 inches of mercury at about 100xc2x0 C. for 12 hours. Preferably, reducing the ruthenium salt to elemental ruthenium comprises the steps of (a) heating the catalyst from 25xc2x0 C. to 400xc2x0 C. at a rate of about 0.5xc2x0 C. per minute under a flow of a gas consisting of 10 volume percent of hydrogen in helium at a rate of about 30 ml per minute; (b) maintaining the catalyst at 400xc2x0 C. and changing the gas to pure hydrogen; (c) reducing the catalyst in the pure hydrogen for about 16 hours; and cooling the catalyst under a helium flow to room temperature. Preferably, passivating the catalyst by placing the reduced catalyst in a stream of about 1 to 10, preferably between 1 and 3, volume percent of oxygen in argon or other inert gas at room temperature for about 1 hour. For using the composition for the conversion of lactic acid to propylene glycol, it is preferable that the ruthenium is reduced in a reaction vessel with hydrogen prior to contact with a reaction mixture.
Therefore, it is an object of the present invention to provide a process for producing propylene glycol in a reaction mixture using a substrate that is derived from renewable resources. In particular, a process which uses lactic acid which can be produced inexpensively in fermentation cultures.
It is also an object of the present invention to provide a process for producing propylene glycol from lactic acid which has high selectivity and yield.
It is further an object of the present invention to provide a composition that is a catalyst for the conversion of lactic acid to propylene glycol that effects the conversion at high yields and selectivities.